(1) Field of the Invention
The present invention relates to a thermoplastic polyester composition having an excellent gas barrier property and an improved flavor-retaining property, and a vessel formed therefrom. More particularly, the present invention relates to a thermoplastic polyester composition having an excellent gas barrier property, in which by reducing the concentration of acetaldehyde contained in the polyester, the storage property of a food or the like and the flavor-retaining property and fragrance-retaining property are highly improved, and a vessel formed from this thermoplastic polyester composition.
(2) Description of the Prior Art
Since polyethylene terephthalate is excellent in mechanical properties such as formability and creep resistance and can be biaxially molecularly oriented, polyethylene terephthalate is widely used for the production of light-weight plastic vessels excellent in creep resistance, impact resistance, rigidity, gas barrier property, light weight and transparency, especially bottles for drinks. However, the gas permeability of a polyester bottle is much higher than that of a glass bottle and cannot be neglected. For example, it is said that in case of a small bottle having a volume smaller than 1 liter, which is filled with a carbonated drink such as cola, the storage period is about 2 months at longest.
Thermoplastic polyesters excellent in the gas barrier property have already been proposed and known. For example, in the specification of U.S. Pat. No. 4,398,017, it is taught that a copolyester comprising terephthalic acid and isophthalic acid as the acid component and etheylene glycol and bis (2-hydroxyethyoxy)benzene as the diol component is excellent in the gas barrier property.
However, a thermoplastic polyester is defective in that at the heat-molding step, acetaldehyde is formed by thermal decomposition of the polyester and when the thermoplastic polyester is formed into a vessel, this acetaldehyde in the vessel wall migrates into a content and the flavor or fragrance of the content is degraded.
The concentration of acetaldehyde contained in the polyester is especially high in the above-mentioned gas barrier polyester, and it was confirmed that the aldehyde concentration in the above-mentioned gas barrier polyester is 2 to 5 times as high as in polyethylene terephthalate. Accordingly, if the gas barrier polyester is used as a constituent material of a vessel, permeation of gases such as oxygen and carbon dioxide gas and permeation of flavor and aroma components can be controlled, but because of migration of acetaldehyde into the content, it is impossible to attain the intended effects sufficiently.